Advances in the Design of Dye-Sensitized Solar Cells
Dayne Swearer
Drexel University - Department of Chemistry
Chem 767 Fall 2012
Contents
I. Introduction
II. What are Dye-Sensitized Solar Cells?
III. Designs of Dye-Sensitized Solar Cells
IIIa. N-type Metal Oxide Semiconductors
IIIb. Photosensitive Molecular Dyes
IIIc. Redox Active Electrolytes
IV. Conclusions
V. References
Introduction
The direst problem facing the world’s population today is finding a sustainable alternative to wean our exponentially growing population off of fossil fuels. The most promising sustainable alternative is -- without a doubt -- solar energy. The sun provides more than an abundant amount of energy to the Earth’s surface. The exploitation of this reliable energy source with cheap and easy to manufacture materials will be a crowning achievement for our society.
Devices designed for the capture of solar radiation have been known for over 125 years [1]. By the middle of the 20th century silicon solar cells were realized as the leader in the photovoltaic market. Silicon photovoltaics contain both photo absorption and charge separation responsibilities in either mono- or poly-crystalline Si materials. These crystalline materials are very fragile and therefore use is limited rigid substrates. Today typical commercially available Si photovoltaic’s are between 12-15% efficient, while the highest efficiency laboratory cells are approaching the theoretical limit for power efficiency. Despite high efficiencies of these cells ultra-pure Si is mandatory and subsequently requires extremely energy intensive processes for their production.
The need for efficient, flexible, and cheap solar cells led to the development of nanocrystalline dye-sensitized solar cells (DSC) by O’Regan and Gratzel in 1991 [2]. These photovoltaics are based on charge separation into different materials, rather than having all requirements for electrical conversion of solar energy by one material. The idea of charge separation dates back to the late 1960s [3] and the idea of using this idea for electric generation dates back to 1972 [4]. The paper by O’Regan and Gratzel showed the true potential of these cells, with an efficiency of 7.9% and hence over the past twenty years research on dye-sensitized solar cells has grown exponentially.
II. What are Dye-Sensitized Solar Cells?
DSCs are a type of thin-film solar cells. This design allows for greater flexibility for where this type of solar cell can be used. Other types of thin film designs include cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and quantum dot solar cells (QDSCs). Pure organic/polymer solar cells are also of research interests, with certified efficiencies of 8.3% [5]. Each of these types of solar cells has highly active researchers investigating them, yet DSC’s remain the cheapest and most efficient of the competing thin-film photovoltaic designs.
A DSC differs from a conventional Si photovoltaic by separating light absorption and charge transport into different materials as shown in Fig. 1. In a DSC light is absorbed at a redox active organic or organometallic dye. The dye injects an electron into an n-type metal oxide layer, which is restored by an electron donated from a redox electrolyte solution or p-type semiconductor causing the formation of a positive “hole” in the electrolyte. The electron and hole travel through the metal oxide and redox electrolyte, respectively, to the front and back contacts of the cell, which is collected as electrical current. The redox electrolyte is reduced at the cathode, by the electrons from the external load. The overall goal of a DSC is to generate electrical power from solar energy through equilibrium of redox chemistry across several chemical species.
Figure 1. Schematic of a mesoporous titania based dye-sensitized solar cell. Original source was reference #2.
III. Design of a Dye-Sensitized Solar Cell
With the separation of functions into several different materials in the cell there are numerous areas of research for the improvement of DSCs. Stability of the cells under long-term illumination is a key consideration in the design and construction of new materials for DSCs. A photovoltaic device should be stable for up to 20 years without performance degradation. Given these terms, all constituents of DSCs are under strict scrutiny for their performance. Transparent conducting glasses and electrodes remain an area of research interest for DSCs, however the majority of research has gone into the “working” parts of the cell such as the n-type semiconductor, photosensitive dye, and redox electrolyte.
IIIa. N-type Metal Oxide Semiconductors
The longstanding material of choice for the n-type semiconductor for dye-sensitized solar cells since the original report in 1991 has been Titanium Dioxide (TiO2) also known as titania. Other materials such as ZnO, Nb2O5 and SnO2 have been investigated but TiO2 still holds monopoly on the use in DSCs because of the low cost, and ideal width of the band-gap in this material [6]. Historic attempts at semiconductor sensitization by organic molecules frequently used a thin compact layer of TiO2 to absorb dyes, and were often found to have sub-par incident photon conversion efficiencies. A breakthrough occurred when TiO2 nanoparticles were sintered together to form a “fractal” geometry forming a mesopourous network as shown in Fig. 2. This mesoporous electrode performs quite differently than compact layers, and is believed to be essential for efficient DSCs.
Figure 2. Scanning electron micropse (SEM) image of mesoporous titania film formed by sintering 20nm nanoparticles. Original image from reference #6.
The idea of nanostructuring n-type metal oxides has seen a high level of interest among the international research community in recent years. Many types of nanostructures have been implemented into DSCs to date including simple geometries to nanorods and nanotubes [7]. The idea behind this structural amendment is that one-dimensional nanostructures will facilitate pore diffusion and allow charge transfer junctions to form with better control [8].
IIIb. Photosensitive Molecular Dyes
By far the most widely augmented parts of dye-sensitized solar cells are the photoactive sensitizing dye and the redox electrolyte. The most popular sensitizing dyes are based on Ruthenium organometallic complexes, but several other non-Ru organometallic complexes, organic, and complex donor-p-bridge-acceptor (D-p-A) compounds. In the groundbreaking 1991 paper by O’Regan and Gratzel they used a polynuclear Ru-complex originally developed as an antenna-sensitizer. Over time many more Ru sensitizers were developed, however many of the key features from the original dye such as anchoring groups (carboxylate, phosphonate, or hydroxamate) and short lifetime of metal-to-ligand charge transfer (MLCT). One paramount dye discovered in 1993, the N3 dye [9], was unmatched in performance for several years until in 2001 N749 or the “black dye” achieved a record power conversion efficiency of 10.4% [10]. One major goal for molecular engineering of efficient sensitizers is pushing absorption into longer wavelengths. By extending the conjugation in the black dye from a bipyridine to a terpyridine ligand the photo response of the black dye extends 100nm further into the IR. Ruthenium complexes of systems of higher conjugation such as quaterpyridyl complexes are expected to yield promising results in the upcoming years [11].
Figure 3. The absorbtions of N3 (shown as red structure) and the N749 (green structure). Original image from reference #6.
In the design of new sensitizer dyes it is important to not overlook the importance of auxiliary anchoring ligands, but there is a need to have a larger percent of molecular absorption towards the IR. By keeping one dicarboxy-bipyridine ligand on the Ru and substituting with 2-(hexathio)-thiophene moieties significant red shifts can be achieved as seen in the C101 and Z991 dyes. The introduction of thiophene directly to the bipyridine ligands also raises the extinction coefficients to values much higher than N3 and N749 dyes. This increase can allow for the minimization of TiO2 thickness, which decreases the resistance, the electrolyte experiences when filling the mesopourous layer and return to the counter electrode for reduction. This increase in speeds leads to higher efficiency. Synthetic variants of this model presently reach 12% efficiency and show promise for practical use [12].
While the vast majority of dyes are based on asymmetric architectures, a new paradigm was presented in 2009 of cyclometalated Ruthenium complexes [13]. These dyes remove the isothiocyanate ligands, which are considered the weakest link in the battle against chemical degradation of this type of photosensitive dyes. Under certain conditions the sulfur atom can be reduced rather than the metal center and the loss of ionic sulfur will occur, disrupting future redox cycles. The promising YE05 dye demonstrates a significant red shift over isothiocyanate containing dyes such as N3. The stronger donor properties of the replacement ligand narrow the gap between the HOMO and LUMO of the sensitizer, and efficiency with this sensitizer is reported at 10.1% efficiency.
Purely organic sensitizers have also been vastly explored [14]. The removal of expensive and limited precious metals from sensitizers would greatly increase potential for large-scale implementation of these solar cells. Progress has been made quick in this particular area. The first reports of purely organic sensitizers in 1998 had a laughable 1.3% efficiency. Within five years efficiency had jumped to 7.7% based on coumarin backbones with thiophene moities acting as a p-bridge to carboxylic anchors. In another five years the efficiency jumped again to 9.7% a decade later by the C217 dye [15].
Figure 4. Chemical structures of organic sensitizers developed by Zhang et. al. Sensitizers designed on D-pi-bridged-A principles. C217 shows the one of the highest incident proton conversion efficiencies of a pure organic sensitizer to date. Original image from reference #15.
In 2011, the highest power efficiency to date was measured from a porphyrin based D-p-A cell. Despite that fact that porphyrin structures are found in naturally in photosynthesis, they are typically not see in DSC applications because of poor absorption in the near IR. The record setting performance was a combination of the D-p-A porphyrin dye designated YD2-o-C8 and the increasingly common Co(II/III)tris(bipyridyl) redox electrolyte. The design of YD2-o-C8 incorporates long-chain alkoxy substituent’s to retard back electron transfer, a process that occurs when electrons injected from dye that reside in the TiO2 reduce the electrolyte at the surface of the oxide. As stated previously, porphyrins have not typically been seen in DSCs because of poor absorption in the near IR, in this structure however the Zn porphyrin center only acts as a highly conjugated aromatic bridge between the electron donor and acceptor. The combination of YD2-o-C8 and Co(II/III)tris(bipyridyl) redox electrolyte shows a power conversion efficiency of 12.3% [16].
Figure 5. Molecular structure of YD2-o-C8, the highest reported efficiency of a photosensitive dye in a mesoporous DSC. Original image from reference #16.
IIIc. Redox Active Electrolytes
Several viable options for redox electrolytes have been proposed. The most common redox electrolyte has been an iodide/triiodide system in which the cation can vary. However, as mentioned above the system has several undesirable properties including absorption on the visible light region of the electromagnetic spectrum, it is highly corrosive, and requires a volatile solvent as a carrier. While the iodide/triiodide redox couple has been by far the most widely used electrolyte system a large attention has been paid to developing alternatives without these particular drawbacks.
Work done by Wang et. al. has developed another organic electrolyte system which does not corrode metal contacts of the electrodes within the cell and is does not absorb any significant fraction of visible light. This particular electrolyte system uses a disulide/thiolate system based on 1-methyltetrazole and its disulide-bridged dimer. The redox potential was determined to be 0.485V against a normal hydrogen electrode (NHE). The measurements for redox potential of the much more common iodide redox electrolyte species range between 0.4V and 0.53V depending on which solvent the electrolyte was measured in. The reason for this systems stability is the delocalization of the thiolate anion over the four nitrogens in the ring, and has been contributed to be a major factor in its stability. Unfortunately, despite extremely similar redox properties this disulfide/thiolate system still requires volatile aprotic solvents such as DMF for proper function. The use of volatile solvents is a major problem that scientists and engineers are trying to get away from. Disulfide/thiolate redox systems are extremely promising however, especially for scale up practicalities because of their non-corrosive properties. Current incident proton conversion efficiencies of this system are 6.4% [17].
More recently redox systems based on colbalt polypyridyl systems have gained attention because of many improved properties of iodide/triiodide [18]. Similar to the disulide/thiolate system discussed above, colbalt polypyridyl systems show negligible absorption in the 390-480nm region, the area which triiodide absorbs. Another distinct characteristic of the colbalt polypyridyl system is dramatic increase in the oxidation potential, and therefore closer match to the oxidation potential of the photosensitive dye employed in these systems. Colbalt electrolyte systems have historically shown very little corrosion on metal electrodes.
Organic electrolyte systems frequently encounter the problem of a necessary carrier solvent, which adds volatility to the system and therefore reduces cell lifetime. One answer for removing this factor from the stability equation almost completely is the use of ionic liquids as the electrolyte in DSC systems [19]. These interesting materials boil down to essentially being salts in a liquid form. As salts, many known ionic liquids have extremely high vapor pressures and are effectively non-volatile. In addition, ionic bonding mechanisms allow for extremely fast electron transfer reactions to occur. Oxidation potential can be easily tuned by exchanging either the anion or cation of the system.
There have been several attempts to completely remove the liquid electrolyte from the system and replace with a solid-state system. One of the most promising attempts has been reported by researchers at the Northwestern University using an p-type inorganic semiconductor CsSnI3 yielding 10.2% incident proton conversion efficiency [20]. These cells completely replace liquid electrolytes, however, unlike other attempts at engineering efficient redox electrolytes this new CsSnI3 system absorbs large portions of the red and near IR. Cells using this new electrolyte were actually completely function without the incorporation of a dye; very low conversion efficiencies were reported of ~0.2%. The use of a solid p-type semiconductor hold many promises for removing some of the stability problems typically associated with liquid electrolyte systems.
IV. Conclusion
The large number of advances made in this field over the past two decades push closer to the reality of finding more sustainable, economically viable, and highly efficient solar cells. Depending on your source the exact numbers on the “bottom line” for solar cell efficiency will vary, but many will agree a baseline of 10% efficiency is needed for commercial application. When this standard was reached the ball started rolling on the industrialization in the late 2000s, however even today we are far from complete outfitting of this technology. Without a doubt this field will continue to grow, and at the rate of current discovery, dye-sensitized solar cells will be an integral part of everyone’s live all around the world.
V. References
Weston, E. (1888). Apparatus for Utilizing Solar Radiant Energy. PSWFILE
O’Regan, B., & Gratzel, M. (1991). A low cost, high-efficiency based on dye sensitized colloidal TiO2 films. Nature, 353, 737-739. DOI:
Gerischer, H. (1968). Sensitization Of Charge Injection into Semiconductors with Large Band Gap. Electrochemica Acta, 1509-1515. DOI:
Tributsch, H. (1972). Reaction of Excited Chlorophyll Molecules at Electrodes in Photosynthesis. Photochemistry and Photobiology, 16, 261-269. PSWFILE
Zhicai, H. (2012). Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nature Photonics, 6, 591-595. DOI:
Grätzel, M. (2003). Dye-sensitized solar cells. Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 4(2), 145-153. DOI:
Bandara, J., Paulose, M., Wietasch, H., Varghese, O. K., Mor, G. K., Latempa, T. J., Thelakkat, M., et al. (2008). Highly Efficient Solar Cells using TiO 2 Nanotube Arrays Sensitized with a Donor-Antenna Dye. Nano Letters, 6, 1654-1659. DOI:
Hosono, E., Fujihara, S., Honma, I., & Zhou, H. (2005). The Fabrication of an Upright-Standing Zinc Oxide Nanosheet for Use in Dye-Sensitized Solar Cells. Advanced Materials, 17(17), 2091-2094. DOI:
Nazeeruddin, M. K., Kay, A., Miiller, E., Liska, P., Vlachopoulos, N., Gratzel, M., Lausanne, C.-, et al. (1993). Conversion of Light to Electricity by Charge Transfer Sensitizers on Nanocrystalline Ti02 Electrodes. Journal of the American Ceramic Society, 115(4), 6382-6390. DOI:
Nazeeruddin, M. K., Péchy, P., Renouard, T., Zakeeruddin, S. M., Humphry-Baker, R., Comte, P., Liska, P., et al. (2001). Engineering of efficient panchromatic sensitizers for nanocrystalline TiO(2)-based solar cells. Journal of the American Chemical Society, 123(8), 1613-24. DOI:
Renouard, T., Fallahpour, R.-a, Nazeeruddin, M. K., Humphry-Baker, R., Gorelsky, S. I., Lever, a B. P., & Grätzel, M. (2002). Novel ruthenium sensitizers containing functionalized hybrid tetradentate ligands: synthesis, characterization, and INDO/S analysis. Inorganic chemistry, 41(2), 367-78. DOI:
Cao, Y., Bai, Y., Yu, Q., Cheng, Y., Liu, S., Shi, D., & Gao, F. (2009). Dye-Sensitized Solar Cells with a High Absorptivity Ruthenium Sensitizer Featuring a 2- ( Hexylthio ) thiophene Conjugated Bipyridine. Journal of Physical Chemistry: C, 113, 6290-6297. DOI:
Bessho, T., Yoneda, E., Yum, J.-H., Guglielmi, M., Tavernelli, I., Imai, H., Rothlisberger, U., et al. (2009). New paradigm in molecular engineering of sensitizers for solar cell applications. Journal of the American Chemical Society, 131(16), 5930-4. DOI:
Mishra, A., Fischer, M. K. R., & Bäuerle, P. (2009). Metal-free organic dyes for dye-sensitized solar cells: from structure: property relationships to design rules. Angewandte Chemie (International ed. in English), 48(14), 2474-99. DOI:
Zhang, G., Bala, H., Cheng, Y., Shi, D., Lv, X., Yu, Q., & Wang, P. (2009). High efficiency and stable dye-sensitized solar cells with an organic chromophore featuring a binary pi-conjugated spacer. Chemical communications (Cambridge, England), (16), 2198-200. DOI:
Yella, A., Lee, H.-W., Tsao, H. N., Yi, C., Chandiran, A. K., Nazeeruddin, M. K., Diau, E. W.-G., et al. (2011). Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency. Science (New York, N.Y.), 334(6056), 629-34. DOI:
Wang, M., Chamberland, N., Breau, L., Humphry-baker, R., Zakeeruddin, S. M., & Gra, M. (2010). An Organic Redox Electrolyte to rival triiodide / iodide in dye-sensitized solar cells. Nature Chemistry, 2(April), 385-389. DOI:
Yum, J.-H., Baranoff, E., Kessler, F., Moehl, T., Ahmad, S., Bessho, T., Marchioro, A., et al. (2012). A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials. Nature communications, 3, 631. DOI:
Hiroshi Matsui, Kenichi Okada, Nobuo Tanabe, Ryuji Kawano, M. W. (2004). Dye-Sensitized Solar Cells Using Ionic Liquid-based electrolytes. Transactions of the Materials Research Society of Japan, 29, 1017-1020. PSWFILE
Chung, I., Lee, B., He, J., Chang, R. P. H., & Kanatzidis, M. G. (2012). All-solid-state dye-sensitized solar cells with high efficiency. Nature, 485(7399), 486-9. DOI:
Advances in the Design of Dye-Sensitized Solar Cells
Dayne Swearer
Drexel University - Department of Chemistry
Chem 767 Fall 2012
Contents
I. Introduction
II. What are Dye-Sensitized Solar Cells?
III. Designs of Dye-Sensitized Solar Cells
IIIa. N-type Metal Oxide Semiconductors
IIIb. Photosensitive Molecular Dyes
IIIc. Redox Active Electrolytes
IV. Conclusions
V. References
Introduction
The direst problem facing the world’s population today is finding a sustainable alternative to wean our exponentially growing population off of fossil fuels. The most promising sustainable alternative is -- without a doubt -- solar energy. The sun provides more than an abundant amount of energy to the Earth’s surface. The exploitation of this reliable energy source with cheap and easy to manufacture materials will be a crowning achievement for our society.
Devices designed for the capture of solar radiation have been known for over 125 years [1]. By the middle of the 20th century silicon solar cells were realized as the leader in the photovoltaic market. Silicon photovoltaics contain both photo absorption and charge separation responsibilities in either mono- or poly-crystalline Si materials. These crystalline materials are very fragile and therefore use is limited rigid substrates. Today typical commercially available Si photovoltaic’s are between 12-15% efficient, while the highest efficiency laboratory cells are approaching the theoretical limit for power efficiency. Despite high efficiencies of these cells ultra-pure Si is mandatory and subsequently requires extremely energy intensive processes for their production.
The need for efficient, flexible, and cheap solar cells led to the development of nanocrystalline dye-sensitized solar cells (DSC) by O’Regan and Gratzel in 1991 [2]. These photovoltaics are based on charge separation into different materials, rather than having all requirements for electrical conversion of solar energy by one material. The idea of charge separation dates back to the late 1960s [3] and the idea of using this idea for electric generation dates back to 1972 [4]. The paper by O’Regan and Gratzel showed the true potential of these cells, with an efficiency of 7.9% and hence over the past twenty years research on dye-sensitized solar cells has grown exponentially.
II. What are Dye-Sensitized Solar Cells?
DSCs are a type of thin-film solar cells. This design allows for greater flexibility for where this type of solar cell can be used. Other types of thin film designs include cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and quantum dot solar cells (QDSCs). Pure organic/polymer solar cells are also of research interests, with certified efficiencies of 8.3% [5]. Each of these types of solar cells has highly active researchers investigating them, yet DSC’s remain the cheapest and most efficient of the competing thin-film photovoltaic designs.
A DSC differs from a conventional Si photovoltaic by separating light absorption and charge transport into different materials as shown in Fig. 1. In a DSC light is absorbed at a redox active organic or organometallic dye. The dye injects an electron into an n-type metal oxide layer, which is restored by an electron donated from a redox electrolyte solution or p-type semiconductor causing the formation of a positive “hole” in the electrolyte. The electron and hole travel through the metal oxide and redox electrolyte, respectively, to the front and back contacts of the cell, which is collected as electrical current. The redox electrolyte is reduced at the cathode, by the electrons from the external load. The overall goal of a DSC is to generate electrical power from solar energy through equilibrium of redox chemistry across several chemical species.
III. Design of a Dye-Sensitized Solar Cell
With the separation of functions into several different materials in the cell there are numerous areas of research for the improvement of DSCs. Stability of the cells under long-term illumination is a key consideration in the design and construction of new materials for DSCs. A photovoltaic device should be stable for up to 20 years without performance degradation. Given these terms, all constituents of DSCs are under strict scrutiny for their performance. Transparent conducting glasses and electrodes remain an area of research interest for DSCs, however the majority of research has gone into the “working” parts of the cell such as the n-type semiconductor, photosensitive dye, and redox electrolyte.
IIIa. N-type Metal Oxide Semiconductors
The longstanding material of choice for the n-type semiconductor for dye-sensitized solar cells since the original report in 1991 has been Titanium Dioxide (TiO2) also known as titania. Other materials such as ZnO, Nb2O5 and SnO2 have been investigated but TiO2 still holds monopoly on the use in DSCs because of the low cost, and ideal width of the band-gap in this material [6]. Historic attempts at semiconductor sensitization by organic molecules frequently used a thin compact layer of TiO2 to absorb dyes, and were often found to have sub-par incident photon conversion efficiencies. A breakthrough occurred when TiO2 nanoparticles were sintered together to form a “fractal” geometry forming a mesopourous network as shown in Fig. 2. This mesoporous electrode performs quite differently than compact layers, and is believed to be essential for efficient DSCs.
The idea of nanostructuring n-type metal oxides has seen a high level of interest among the international research community in recent years. Many types of nanostructures have been implemented into DSCs to date including simple geometries to nanorods and nanotubes [7]. The idea behind this structural amendment is that one-dimensional nanostructures will facilitate pore diffusion and allow charge transfer junctions to form with better control [8].
IIIb. Photosensitive Molecular Dyes
By far the most widely augmented parts of dye-sensitized solar cells are the photoactive sensitizing dye and the redox electrolyte. The most popular sensitizing dyes are based on Ruthenium organometallic complexes, but several other non-Ru organometallic complexes, organic, and complex donor-p-bridge-acceptor (D-p-A) compounds. In the groundbreaking 1991 paper by O’Regan and Gratzel they used a polynuclear Ru-complex originally developed as an antenna-sensitizer. Over time many more Ru sensitizers were developed, however many of the key features from the original dye such as anchoring groups (carboxylate, phosphonate, or hydroxamate) and short lifetime of metal-to-ligand charge transfer (MLCT). One paramount dye discovered in 1993, the N3 dye [9], was unmatched in performance for several years until in 2001 N749 or the “black dye” achieved a record power conversion efficiency of 10.4% [10]. One major goal for molecular engineering of efficient sensitizers is pushing absorption into longer wavelengths. By extending the conjugation in the black dye from a bipyridine to a terpyridine ligand the photo response of the black dye extends 100nm further into the IR. Ruthenium complexes of systems of higher conjugation such as quaterpyridyl complexes are expected to yield promising results in the upcoming years [11].
In the design of new sensitizer dyes it is important to not overlook the importance of auxiliary anchoring ligands, but there is a need to have a larger percent of molecular absorption towards the IR. By keeping one dicarboxy-bipyridine ligand on the Ru and substituting with 2-(hexathio)-thiophene moieties significant red shifts can be achieved as seen in the C101 and Z991 dyes. The introduction of thiophene directly to the bipyridine ligands also raises the extinction coefficients to values much higher than N3 and N749 dyes. This increase can allow for the minimization of TiO2 thickness, which decreases the resistance, the electrolyte experiences when filling the mesopourous layer and return to the counter electrode for reduction. This increase in speeds leads to higher efficiency. Synthetic variants of this model presently reach 12% efficiency and show promise for practical use [12].
While the vast majority of dyes are based on asymmetric architectures, a new paradigm was presented in 2009 of cyclometalated Ruthenium complexes [13]. These dyes remove the isothiocyanate ligands, which are considered the weakest link in the battle against chemical degradation of this type of photosensitive dyes. Under certain conditions the sulfur atom can be reduced rather than the metal center and the loss of ionic sulfur will occur, disrupting future redox cycles. The promising YE05 dye demonstrates a significant red shift over isothiocyanate containing dyes such as N3. The stronger donor properties of the replacement ligand narrow the gap between the HOMO and LUMO of the sensitizer, and efficiency with this sensitizer is reported at 10.1% efficiency.
Purely organic sensitizers have also been vastly explored [14]. The removal of expensive and limited precious metals from sensitizers would greatly increase potential for large-scale implementation of these solar cells. Progress has been made quick in this particular area. The first reports of purely organic sensitizers in 1998 had a laughable 1.3% efficiency. Within five years efficiency had jumped to 7.7% based on coumarin backbones with thiophene moities acting as a p-bridge to carboxylic anchors. In another five years the efficiency jumped again to 9.7% a decade later by the C217 dye [15].
In 2011, the highest power efficiency to date was measured from a porphyrin based D-p-A cell. Despite that fact that porphyrin structures are found in naturally in photosynthesis, they are typically not see in DSC applications because of poor absorption in the near IR. The record setting performance was a combination of the D-p-A porphyrin dye designated YD2-o-C8 and the increasingly common Co(II/III)tris(bipyridyl) redox electrolyte. The design of YD2-o-C8 incorporates long-chain alkoxy substituent’s to retard back electron transfer, a process that occurs when electrons injected from dye that reside in the TiO2 reduce the electrolyte at the surface of the oxide. As stated previously, porphyrins have not typically been seen in DSCs because of poor absorption in the near IR, in this structure however the Zn porphyrin center only acts as a highly conjugated aromatic bridge between the electron donor and acceptor. The combination of YD2-o-C8 and Co(II/III)tris(bipyridyl) redox electrolyte shows a power conversion efficiency of 12.3% [16].
IIIc. Redox Active Electrolytes
Several viable options for redox electrolytes have been proposed. The most common redox electrolyte has been an iodide/triiodide system in which the cation can vary. However, as mentioned above the system has several undesirable properties including absorption on the visible light region of the electromagnetic spectrum, it is highly corrosive, and requires a volatile solvent as a carrier. While the iodide/triiodide redox couple has been by far the most widely used electrolyte system a large attention has been paid to developing alternatives without these particular drawbacks.
Work done by Wang et. al. has developed another organic electrolyte system which does not corrode metal contacts of the electrodes within the cell and is does not absorb any significant fraction of visible light. This particular electrolyte system uses a disulide/thiolate system based on 1-methyltetrazole and its disulide-bridged dimer. The redox potential was determined to be 0.485V against a normal hydrogen electrode (NHE). The measurements for redox potential of the much more common iodide redox electrolyte species range between 0.4V and 0.53V depending on which solvent the electrolyte was measured in. The reason for this systems stability is the delocalization of the thiolate anion over the four nitrogens in the ring, and has been contributed to be a major factor in its stability. Unfortunately, despite extremely similar redox properties this disulfide/thiolate system still requires volatile aprotic solvents such as DMF for proper function. The use of volatile solvents is a major problem that scientists and engineers are trying to get away from. Disulfide/thiolate redox systems are extremely promising however, especially for scale up practicalities because of their non-corrosive properties. Current incident proton conversion efficiencies of this system are 6.4% [17].
More recently redox systems based on colbalt polypyridyl systems have gained attention because of many improved properties of iodide/triiodide [18]. Similar to the disulide/thiolate system discussed above, colbalt polypyridyl systems show negligible absorption in the 390-480nm region, the area which triiodide absorbs. Another distinct characteristic of the colbalt polypyridyl system is dramatic increase in the oxidation potential, and therefore closer match to the oxidation potential of the photosensitive dye employed in these systems. Colbalt electrolyte systems have historically shown very little corrosion on metal electrodes.
Organic electrolyte systems frequently encounter the problem of a necessary carrier solvent, which adds volatility to the system and therefore reduces cell lifetime. One answer for removing this factor from the stability equation almost completely is the use of ionic liquids as the electrolyte in DSC systems [19]. These interesting materials boil down to essentially being salts in a liquid form. As salts, many known ionic liquids have extremely high vapor pressures and are effectively non-volatile. In addition, ionic bonding mechanisms allow for extremely fast electron transfer reactions to occur. Oxidation potential can be easily tuned by exchanging either the anion or cation of the system.
There have been several attempts to completely remove the liquid electrolyte from the system and replace with a solid-state system. One of the most promising attempts has been reported by researchers at the Northwestern University using an p-type inorganic semiconductor CsSnI3 yielding 10.2% incident proton conversion efficiency [20]. These cells completely replace liquid electrolytes, however, unlike other attempts at engineering efficient redox electrolytes this new CsSnI3 system absorbs large portions of the red and near IR. Cells using this new electrolyte were actually completely function without the incorporation of a dye; very low conversion efficiencies were reported of ~0.2%. The use of a solid p-type semiconductor hold many promises for removing some of the stability problems typically associated with liquid electrolyte systems.
IV. Conclusion
The large number of advances made in this field over the past two decades push closer to the reality of finding more sustainable, economically viable, and highly efficient solar cells. Depending on your source the exact numbers on the “bottom line” for solar cell efficiency will vary, but many will agree a baseline of 10% efficiency is needed for commercial application. When this standard was reached the ball started rolling on the industrialization in the late 2000s, however even today we are far from complete outfitting of this technology. Without a doubt this field will continue to grow, and at the rate of current discovery, dye-sensitized solar cells will be an integral part of everyone’s live all around the world.
V. References